Shan Wenrui Niu Zhilei Yanping Guo The distribution of zeros of solutions for a class of neutral delay differential equations. 1137-1150 2007 186 Applied Mathematics and Computation 2 http://dx.doi.org/10.1016/j.amc.2006.08.056 db/journals/amc/amc186.html#WenruiZY07